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I am eagerly awaiting the next annoncement where
someone again finds evidence to refute the dark matter
claims. It seems like the science; "Dark Matter
is like this" - "No, it can't be, actually it's
like that". Is not going to end soon.

Join me. Come to the dark side, and together, we
will expand the universe.

As others have said, accelerating expansion means that objects very distant are moving away from us faster than closer objects are moving away from us. If you have time for some interesting reading, I'd recommend a title called Atom [amazon.com] which is very readable and is a good primer on theory from the big bang to present time. It won't answer many questions about dark energy, but if anything, it'll give you a good idea of what we know in very readable terms and most likely get you to want to read more:) It w

I am eagerly awaiting the next annoncement where someone again finds evidence to refute the dark matter claims. It seems like the science; "Dark Matter is like this" - "No, it can't be, actually it's like that".

The only articles I've seen that make statements like that are the commentaries on the commentaries on the dumbed-down press releases on the actual publications.

What's actually been happening is more along the lines of:

"There's a discrepency between galactic models and observations. What did we get wrong in the model/what needs to be added?"

"Our models only work if we have B _and_ C, and we've ruled out C1 and C2, but C3 still works."

"What kind of C3?"

"New observations show a new effect in addition to the old one. How do we explain it?"

"Maybe E? Maybe F?"

[Etc.]

This is a process of examining many possible explanations, and weeding out the ones that don't work until we have reasonable confidence that the ones left _do_ work.

We've gone from "galactic rotation doesn't match models based on stars alone, what could be causing this?" to "we know that there's about X amount of normal matter we aren't seeing, Y amount of abnormal matter that we aren't seeing, and that the properties of the abnormal matter fall somewhere in this range (that's wide but being narrowed)". There's surprisingly little backtracking. Tests that detect or fail to produce evidence for dark matter of various types all help to increase our understanding of what dark matter's actual properties are.

As for dark energy, if anything, it would be surprising if something like it _didn't_ exist. We already knew that a scalar field with similar properties was likely present in the early universe, and several models proposed universes where the _absence_ of the field was only a local effect. Even relativity contained a similar type of effect that was set to zero a priori as opposed to forced to zero through a mechanism inherent in the model.

We're still sifting through the myriad of possibilities, but we certainly are learning something each step of the way.

So does anybody have a good,cheap,quick (pick two) primer on Quantum Physics? Something that can explain what we do know, along with the outstanding issues that we don't know?

Not offhand, but a couple of good places to look would be to check the various online bookstores for quantum mechanics textbooks to see what are recent/available and get good reviews, and course pages at various universities to see what textbooks they use and what online resources they have available. Expect to pay $100 or so for a g

It's not that simple. Cosmology is now in a position pretty much similar to that of a butterfly trying to understand the passing of seasons. Moreover, as the name says, "dark" matter/energy is undetectable directly (at least, so far) - and it's quite challenging to figure out a 'simple' theory for something that not only you can't observe directly, but the indirect observations are difficult and not always very accurate.

Anyway, since it's not very likely that the knowledge of dark matter will have a significant impact on the daily life anytime soon, relax and enjoy the (slow-moving) show.

Photons are normally considered to have zero mass, and to be the smallest possible unit of energy.

Check... although photons can have almost any energy. Low-frequency photons (think IR) have low energy, and high-frequency photons (think gamma rays) have high energy.

Yet, they are also "negative", are they not? That is, they move away from their source.

I have no idea what you're saying here. Photons have no charge and no mass. They are not "negative" in any sense of the word I'm familiar with. One of the fundamental properties of photons is that they are always moving at the speed of light - that's why they move away from their source.

Yet, if a photon will be absorbed by some types of objects, bounce off of others, and simply pass through others - it must have some sort of mass.

Why must it? If you begin to study physics seriously, one of the first pre-conceptions you'll have to let go of is that your "common sense" can be trusted to tell you how things behave in the quantum world. Photons have no mass.

Where does a photon go when it's energy is spent?

A typical fate for a photon would be for it to be absorbed by an atom. In the process, the photon's energy is put into raising one of the atom's electrons from a lower energy state to a higher energy state.

There must be a near infinate supply of photons that have no energy or are waiting to aquire it. It would seem that these photons - assuming they do have mass, in the same sense that electrons have a larger mass, could explain both, no?

No. All photons have a non-zero energy which equals something like h * f, where h = Planck's constant and f = the photon's frequency. I may be off by a factor of 2 pi... it's been a long time since I took Modern Physics!

"dark energy" usually refers to that energy that seems to be driving the galaxies away from each other at an accelerating rate. Normally, we would think that due to the mass of the universe, the universal expansion would slow down, just as a baseball slows down if I toss it upwards. Strangely, we see a growing "anti-gravity" (I hesitate to use that phrase around here) or repulsive force that seems to be proportional to the volume of the universe. Almost as if each cubic centimeter of space itself carries a small repulsive force acting on all other cubic centimeters. This is also why the acceleration is dominant now --- earlier in the history of the universe, when it was smaller, the repulsive force was also smaller in magnitude. As the universe expands, the quantity of 'dark energy' also increases with the universe's volume and now overwhelms the attractive gravitational force of all the matter.

Dark matter, on the other hand, is the name confusingly given to a number of unsolved phenomena. By looking at how the outer parts of galaxies rotate, we get a sense of how much matter is in a given galaxy, as well as its distribution. It seems that there is a great deal of matter in the outer regions of galaxies that does not 'glow' like stars do. In addition, by studying how galaxies move in clusters, we strengthen the case for lots of matter existing between galaxies that is invisible to us. The candidates for this dark matter are many and varied, from innumerable Jupiter-sized objects to cold white dwarfs to small black holes. Current observations are undertaken to rule in or out some of these. Even so, standard Big Bang theory predicts an upper limit to the amount of "ordinary" (baryonic) matter present, so it is possible that some of this dark matter might be weird stuff.

E=mc^2 doesn't mean that mass and energy are the same, it means that's the conversion you use when you convert between them. So if you could turn two 500nm photons into two massive particles (you can't turn a single photon into particles because of conservation of momentum), you could create two 4.4x10^-36 kg particles at rest in the center of mass frame.

It also means that photons do act as a source of gravity, with a strength equal to something with a mass of E/c^2. But in the current universe, their gravitational effect is tiny compared to the gravity of the mass... as a little exercise, try calculating the equivalent rest mass of the entire luminosity of the Milky Way and compare it to the mass of the moon.:-)

I think it is common for people to make up something that helps fill gaps in science. sometimes it turns out right many times it turns out wrong. many times this happens such as space ether [tu-harburg.de].

When we can't explain something we are sometimes better off makeing something up that fills the gap until we can find the more correct answer. There is no such thing as exact science. Only reproduceable observation which eventually becomes accepted fact. Although there is no reason for it to always stay fact if someone says, "Hey, I tried to do the experiment and used this method to test it and I got a diffrent observation!" Well, now it's time to re think that scientific fact.

What happens typically is that the person is downplayed as doing something wrong, adding some new variable to the mix, or something that would throw off the observation in some way. Politics in science is as complicated and painful as anywhere.

Maybe someone can explain... But when the CNN article states that the universe is "accelerating", does that mean it's really accelerating? I thought it was decided that the universe's expansion was expanding at the speed of light. So, I would assume that by accelerating they mean growing bigger and not actually accelerating faster than the speed of light. Unless, this Dark Matter is something that can bend the known laws of physics and travel faster than the speed of light?

Don't you get your science from Monty Python movies. It was explained quite succinctly, in the Meaning of Life.

Whenever life get's you down, Mrs. Brown
and things seem hard and tough,
and people are stupid, obnoxious or daft,
and it feels that you've had quite enough---

Just remember that your standing on a planet that's evolving,
revolving at 900 miles per hour.

It's orbiting at 19 miles per second, so it's reckoned,
a sun that is the source of all our power.

The sun, and you and me and all the stars that we can see
are moving at a million miles a day,
in an outer spiral arm at 40,000 miles an hour,
in this galaxy we call the milky way.

The galaxy itself contains 100 million stars,
it's 40 thousand light years side-to-side.
It bulges in the middle 30 thousand light years thick,
but out by us it's just 3000 light years wide.
We're 30 thousand light years from galactic central point,
we go round every two hundred million years
And our galaxy is only one of millions and billions in this amazing and expanding universe.

Musical interlude

The Universe itself keeps on expanding and expanding
in all of the directions it can whizz.
As fast as it can go, the speed of light, you know
A million miles a minute and that's the fastest speed there is.
So remember when your feeling very small and in-secure,
how amazingly unlikely is your birth.
And prey that there's intelligent life, somewhere out in space,
'cause there's bugger-all down here on Earth.

Huh? There's no "edge" or "center". It's like the old points on the surface of a balloon analogy. The surface doesn't have a center or an edge. All points are expanding equally.

The balloon surface analogy is a 2D example. Yes, there's a center and edge to the balloon, but that's in 3D. The surface doesn't have an edge. Similarly in 4D, the universe can have a center and edge, and does when time is the fourth dimension, but that is measured in time and not in 3D space. In that case, current time is t

However, the observable universe, consisting of all locations that could have affected us since the Big Bang given the finite speed of light, is certainly finite. The edge of the cosmic light horizon is 13.7 billion light years distant. The present distance (comoving distance) to the edge of the observable universe is larger, since the universe has been expanding; it is estimated to be about 50 billion light years

Actually, this is an interesting problem, and seems possible. Take the example of two points on a balloon surface. They also follow this "Hubble" equation. To see this imagine three colinear points on the surface (P0, P1, P2) such that P0 and P1 are 10mm apart and P1 and P2 are 10mm apart. Since they are colinear, P0 and P2 are 20mm apart (following the curvature of the surface).

Now blow the balloon up a little more such that P0 and P1 are 1mm further apart, and thus P1 and P2 are also 1mm further apart (P0 and P2 are 2mm further apart). Then (D=distance, dD=change in distance):

So, in theory, you could blow up a balloon such that two points are moving faster than c relative to each other (V=c=D*K). Let's see how to do this. The distance between any two points on the surface is D = r*Q (r=balloon radius, Q = angle between the points in radians which stays constant as the balloon expands). The change in distance over time is

dD/dt = V = dr/dt*Q.

The furthest two points can get apart is Q=pi (opposite points on the balloon), hence the fastest relative velocity will be between these points. Let V = c and solve:

dr/dt = V/Q = c/pi

In other words, if the radius of the balloon was expanding at a rate of just under 1/3 the speed of light, two points on the balloon would be moving relative to each other at the speed of light. (This would not only take a lot of air, but the rate of air required would go up with the cube of the radius, so you'd want to do this when the radius is very small.)

Applying this 2D analogy to the 3D universe, it doesn't have to be expanding at the speed of light for two distant points to be moving greater than c relative to each other. But it does have to be expanding above a certain rate to achieve this. If it's expanding slower than this critical rate, no two points can be moving faster than light relative to each other. If it's expanding faster, they can. Since the expansion seems to be accelerating, it seems inevitable that it will happen at some point if it hasn't already.

We should also be able to figure out if it has already happened or when it will. We know the constant H (from the Hubble equation H = V/D). (It's easy to calculate anyway, given the distance to any star and it's measured relative velocity.) If we know the history of the expansion rate we know how big the universe is, i.e., this furthest distance Dmax between any two points. We can then solve the Hubble equation V = H*Dmax and see if it is less than or greater than c.

By the way, I don't think this violates relativity, it doesn't say anything about the rate of expansion of the universe. I think this falls into the "warp" concept of traveling faster than the speed of light, i.e., if you can locally expand the universe fast enough, it appears you are moving away faster than the speed of light, and vice-versa if you can contract it fast enough locally it appears that you are approaching faster than the speed of light. I could be wrong about that though.

Tachyons do it all the time. Literally. Just as us tardyons with real rest mass have the speed of light as the upper limit of our velocities and luxons with no rest mass are always moving at the speed of light, tachyons with imaginary rest mass have the speed of light as the lower limit to their velocities.

Nobody's found any yet, but the math says they should be there and nobody's figured out how to disprove them, either.

The universe can't be expanding at the speed of light. Releativity states that the speed of light is as fast as it gets. So if we were all going the speed of light away from the center of the universe there would be no headroom in the velocity for movement in any other direction. That would mean planet orbits, even walking down the street in a certain direction would be impossible.

Consider for a moment that Einstein might have been wrong? Since we can't actually study anything at the speed of light properly, it's all down to the calculations and even great minds might make errors. So, you never know...

Very true.... but if einstien is wrong that breaks open a whole can of somethin. Consider all the observations that are based on einstein's theories. Possibly even these observations.... definitly something to think about.... preferably by someone smarter then me...

Hell, Newton was "wrong," but it wasn't the end of the world. In fact, the corrections to his theories led to some amazing discoveries. I would imagine that any theories that "prove" Einstein wrong will lead to some amazing technology themselves. Of course such speculation leads to... Science Fiction (I am not saying it would be wrong - remember that flying to the moon used to be scifi).

Releativity states that the speed of light is as fast as it gets.... for matter. Relativity makes no such claims as to the speed limit of space itself.

There are quite a number of valid GR metrics which describe space which expands faster than the speed of light, and in fact, it's thought that it did expand faster than the speed of light during the inflationary period.

Those same metrics are the basis of the Alcubierre metric, one of many ways to generate faster-than-light travel without multiply-connected spacetimes (wormholes). Like most "violate the speed of light" metrics, it requires negative energy density matter, though variations on the metric allow for very tiny amounts of negative energy matter to generate it.

This isn't true. The energy that a particle contains has no upper bound, even if its velocity does. Its velocity is simply asymptotic to c.

True, but irrelevant. Within the Swartzchild radius, nothing can escape a black hole's gravity. Period. Doesn't matter if it is travelling at 99% the speed of light or 99.999999999999999999999999999% the speed of light relative to the black hole, it still can't escape.

The entire universe crammed into a small space would have one hell of a Swartzchild radius, and no

No offense, but that's not how relativity works. The thoery of Relativity posits that all measurements are taken from some frame of reference, and it is impossible for an object to go faster than light for any frame of reference.

So, if I'm on a spaceship going 99.9999999% the speed of light from the frame of reference of the earth. However, from the frame of reference of my spaceship, I'm stationary. Now, I can run as fast as I want in any direction, I can even sit in the back of my space ship with a super-powerful gun that shoots bullets at 99.9999999% the speed of light, and fire a couple rounds towards the front of the ship. From the frame of reference in the ship, the bullets will travel at 99.9999999% the speed of light, even when the ship is travelling at 99.9999999% the speed of light in reference to the earth.

But here is where it gets wierd: an observer on earth will not measure the speed of the bullets to be travelling 199.9999996% (99.9999999%x2) the speed of light, they will be measuring the bullet to be travelling just over 99.9999999% the speed of light.

This is because, from the viewpoint of someone on earth, the space ship will be very short, which means even if it still traverses the length of the ship in the same amount of time as it does from the viewpoint of me on the spaceship, it will not have travelled the same distance, which (since v=d/t) means the bullet didn't travel as fast relative to the spaceship (from the viewpoint of earth) as it did from the viewpoint of someone on the spaceship.

Additionally, from the viewpoint of Earth, time is travelling more slowly on the spaceship, which enhances the effect even more.

It's confusing if you don't have a handle on it, but none the less, this is how the theory of relativity works.

Actually, I'd recommend trying a book written by Einstein called "Relativity" [amazon.com]. I've also heard it referred to as "the short book" because apparently he wrote two, one in laymen's terms, and one filled with math/equations. If you really want to go hard-core, you can read his original papers, but takes a bit of work to get through, and it helps if you have a big physics background and are familiar with Maxwell. "Relativity" isn't too hard to understand, though. Plus, it's generally true that you'll never get such a dead-on explanation of a theory as when you get an explanation from the guy who came up with it. I've met a lot of modern physicists whose grasp on Relativity has been corrupted by hearing poor explanations. No risk of that if you go to the source (Einstein).

It's been theorized that space itself is expanding along a fourth, hyperspatial dimension. This would mean there is no "center" of the universe. Think of the galaxies as dots on the surface of a balloon, and the balloon being blown up. Everything moves away from everything else, but there is no real center you can point to.

Yes, and that 4th dimension would be called time. The "center" of the balloon would be the equivalent of the big bang, but as you say there is no "center" in the normal dimensions, i.e.,

The speed of light is only a restriction upon the velocity of matter/energy within space-time. There is no such restriction upon the expansion or movement of space-time itself. If you think about it in the usual 'rubber sheet' model then this is equivalent to saying that particles on the surface of the sheet can only travel up to c, but that the sheet itself can change without such restrictions.

the Universe can expand FASTER than the speed of light. Relativity says matter and energy can not travel faster than the speed of light through space-time, but when talking about dark energy and the expansion of the universe, we are talking about the expanding of space between galaxies, so the galaxies, relative the their local space are not traveling near the speed of light, but relative to inter galactic space, they are.

I thought it was decided that the universe's expansion was expanding at the speed of light.

No. The expansion of the universe refers to the fact that distant galaxies are moving away from us, and that the farther they are, the faster they are moving. This is expressed by the Hubble constant [utk.edu], which has a value of about 50 km/s/Mpc.

The acceleration of the expansion is reflected as this "constant" increasing with increasing distance.

The acceleration is caused by Dark Energy, not Dark Matter.

Dark Matter is either normal matter or subnuclear matter that makes its presence felt as increased gravity, but is not directly observable.

Yes. The universe is accelerating in its expansion. To say that the universe is expanding "at the speed of light" isn't quite right in a couple of ways. First off, if we look at objects nearby, they are moving away from us at some (quite reasonable) finite speed (i.e. the nearby Virgo Cluster is moving only at ~1000 km/s). Secondly, the somewhat more subtle point is that we generally talk about velocites not exceeding the speed of light; however, this is motion THROUGH space. The expansion of the universe (expansion OF space) doesn't necessarily need to follow this rule. ..

I should also point out that "Dark Matter" and "Dark Energy" are COMPLETELY DIFFERENT THINGS (as far as we know). Astronomers have just named them both "Dark" because they don't know what they are. They both also affect the expansion of the universe, but dark matter is slowing down the expansion of the universe (presumably via gravity) and dark energy is accelerating the expansion of the universe (by some yet-unknown force). Dark Matter is weird, but at least it seems to sortta obey the rules of the universe (i.e. gravity); dark energy is completely unlike anything we've seen before.

(1) the universe is not expanding at the speed of light (I think that it is less)
(2) the space-time fabric of the universe is not an object anyway, so FTL rules do not apply

It is actually possible for 2 objects to move apart faster than the speed of light even though neither is moving FTL compared to the other. This statement seems to be nonsensical, until you realize that the expansion is a 4D effect. Think of the galaxies (in 2D) as though they were on the surface of a balloon (2D). Now imagine the

First, dont't mistake Dark matter [wikipedia.org] and Dark Energy [wikipedia.org]. They are completly different beast, and have only in common (is it true ? no one know) that both are in the number of biggest(together with quantum gravity) misteries of modern physics. Dark matter is a problem of mass distribution in tha galaxies. Dark Energy is a reason why universe expansion accelerating. From the formal point of view it's no more than a constant in the equations of General Relativity. And I think you are right, it's not "real" accelera

I think a more accurate answer is in order (not to malign the other posts, as they all contain bits of the complete answer).
Accelerating means that the outer boundary of Universe ("horizon") is moving away from us at an ever increasing rate. The components of the Universe also accelerate away at a proportional rate. It does not simply mean that the Universe is growing bigger (although that is true). It means that it's growing bigger at an increasing rate.
The Universe is *not* expanding at the speed o

This isn't even strictly true. You can violate global speed-of-light travel times without violating local speed-of-light travel times by making space itself move - see the inflationary period, or the Alcubierre metric for more info.

You can imagine it as a speed limit placed on people walking, but there is no such speed limit on moving walkways (like in airports).

And the problem with particles traveling faster than light (tachyonic) is the fact that as they *lose* energy, they go faster, which makes them lose more energy, so they spiral out to infinite speed. Tachyonic modes are unstable, so a theory containing them typically undergoes tachyonic condensation (spontaneous symmetry breaking) and the tachyons gain a positive mass squared.

> A particle, like photon, can travel faster than the speed of light as they are> already traveling at the speed of light to begin with.

I've never heard of the possibility for a photon to travel faster than c, where c is speed of light in vacuum. What is the basis for this? Also, does a photon ever really accelerate anyway? Do bosons and fermions really have the same relativistic physics?

A photon cannot travel faster than light, because it is light. All massless particles that traval at c are called luxons; these include photons, gluons, W and Z particles, and hypothetical gravitons. Anything that travels slower than c (basically all ordinary matter) is a tardyon, and has positive mass. Hypothetical particles that travel faster than light and have negative mass are tachyons. As tardyons accellerate, they gain mass and time slows down for them. As you approach c, mass increases and time

I'd like to think we live on an electron in orbit around the proton of a molecule as part of a giant coffee mug -- our universe is expanding due to some even bigger geek having just poured hot coffee in our universal mug.

A guy I know that works at SLAC as an astrophysicist theorized it's not something that's expanding the universe, it's nothing. He was referring to a possible 'vacuum of space' that is pulling the matter and energy into the...uh, nothingness I guess.

Apparently the astrophysics bunch had evidence about the expanding universe already, I think this helps corroborate other evidence.

But I guess we'll never know for sure until it happens, so I guess we'll have to wait and see.

Does anyone else think that the cutting edge of physics is starting to resemble Ptolemy's system of astronomy? With all this 'dark' energy, and 'dark' matter, it's beginning to look like a lot of hand-waving.

Increasingly complex adjustments [utk.edu](e.g. epicycles) were made to Ptolemy's system to explain the observed motions of the heavenly bodies. Then along comes Copernicus and tells us that we've been looking at it inside out all along, things are simple after all, we just have to adjust our viewpoint.

On the other hand, the decades of precise mesurements by people like Ptolemy provided the data needed to spark an idea in the mind of Copernicus. Science seems to work that way. You have a wonderful complete theory, then a long period of gathering empirical data which conflicts the theory. During this time many cludges are suggested, but real understanding does not come because there isn't enough data yet. Then you reach a point where all the pieces of the puzzle are finally available and 3 people independe

It's possible that Ptolemy had some familiarity with the prior work of Aristarchus [wikipedia.org], who had postlated a heliocentric model of the solar system centuries before. The book in which he did so has been lost (though it probably existed in Ptolemy's time) but correspondence survives discussing it.

This model had been rejected by other philosophers at the time, but the meme was out there even then.

BZZZZT! Dark matter has never been observed directly. Observations have been made of motions (?) that could be explained by the presense of matter other than what we can see. Dark matter and energy are not the only possible explanations. The motions haven't been observed directly either, but infered from other things (red shift perhaps) and I think there are some other assumptions baked in there too. The first I read of dark matter, it went something like this:

The headline to this story is an exaggeration. Of course, you can't blame it on the author seeing as the headlines of the major news sources were exaggerations as well.

So what, we have more evidence the universe is expanding at an accelerating rate. WE ALREADY KNEW THAT! This is just another indication that it's happening. This doesn't "prove" the existence of dark energy. It's still entirely possible (and I would suggest probable) that we just don't know the entire story about gravity. Physicists have gotten gravity wrong before after all.

The original poster has it wrong, more dark matter decreases the expansion of the universe as one would expect, dark energy does the opposite changing the state function of the universe and thus allowing it to expand. IAA astro-physicist

That was proved to me years ago when I met my housemates girlfriend. She was positively festering with it. She radiated me with it so much of it that I now have a latent ability to detect dark energy within a 5m radius.

From the Micro$oft school of cosmology. Take something ( a cosmological model ) that barely works, find out there's a yet another problem with it, and patch it ( with blather about dark matter ) in the hope that it works. Then find out that by introducing the patch to fix one problem, you find another ( er... what exactly is dark matter? Anyone? Please? )

Hey, I rubbished Micro$oft and the whole of modern cosmology in one post. Cool.

There was some PBS special a little while back that talked about "string theory" of reality and the possibility that the "Big Bang" was actually a big "collision" between this and another dimmension. The "collision" or interaction between the two different dimmensions not only created tremendous energy but also left some material from the other dimmesion in this one, dark matter. That is why the stuff is invisible with no known origin but somehow detectable. Kind of like the Old Ones.

Why are they ignoring the obvious (at least to me) possiblity that the universe oscillates around some optimal size. Imagine the universe as a rubber ball. Squeeze the ball and let it go. Every particle inside will immediately start moving away from the others at an accelerated pace, continuing to accelerate until passing the rest boundary, when it will start slowing down. What's causing the expansion? How about the reduction of space curvature? Imagine space as a tablecloth (ok, so I'm knee deep in analogies:) on a table with a hole in the middle. Place a heavy pitcher in the middle and the tablecloth will be pulled through the hole, pulling its edges closer together. This is what happens around a star according to general relativity theory. Now, the star is constantly radiating energy and losing mass, so the space is constantly uncurving. Because it is uncurving, it is expanding. When all the stars burn out, space will start collapsing again as energy falls into black holes. Then the black holes coalesce and make the big bang singularity, which explodes for some reason and everything starts all over again.

I am not certain how this got ranked to 5, but it is a common misconception, so allow me to clarify.

When all the stars burn out, space will start collapsing again as energy falls into black holes.

No. This is not how gravity, according to general relativity, works. The curvature of spacetime is, roughly speaking, proportional to its local mass/energy content. In fact, converting things into black holes doesn't change the curvature of spacetime to any substantial degree once you are more than a few Schwar

Speaking of dark energy, I wonder whether [suppose it exists indeed] dark energy does not break the law of conservation of energy. Once I attended a public talk by someont from Fermi Lab [sorry, cannot recall the name] who said that dark energy is a constant quantity [a very small number in standard units] per volume of space. So, given that the Universe is expanding and is being pushed more and more this way by the dark energy, the quantity of dark energy goes up and up, right? So, if it has indeed the meaning of energy, there is more and more energy in the Universe, contrary to the law of conservation...

Sadly, after the mention public talk only very few questions were allowed and I missed the opportunity to ask the expert in person.

It got moddep up as Interesting +1, but nobody
answered. Maybe this time...

Just a quick reply to this. I'm a graduate student doing computational astrophysics - in particular, cosmological structure formation (galaxies and such). The law of conservation of energy is only valid in closed systems. If the universe isn't a closed system - if there's something 'outside the universe' which is adding/subtracting energy - then energy doesn't necessarily have to be conserved. Also, there are some cosmologists that believe that energy is not conserved on cosmological scales, so the law of conservation of energy is not valid on all scales. I suppose it's fair to say that as of right now, dark energy appears to result in the non-conservation of energy on very large scales, given our current understanding of particle physics. However, there is almost certainly a lot going on that we don't really understand, so it's an open problem.

...I was just wondering if the expansion/contraction might not have something to do with outside forces acting upon the brane (as always, still theory) that our Universe exists in. Think of a piece of rubber sheet with a map of our cosmos on it, then think of it being stretched in different directions, around things, etc. Being stuck in a rather two dimensional viewpoint, we would see contractions and expansions over time, but the time frame may be so great that a very young society (like ours) may not really see the changes.

It may be possible to have a universe that is expanding and contracting at different times based on variables we have no ability to measure, hence never be able to know which way we are going to go, only where we seem to have gone.

couldn't the universe be Like the release of a stretched-out, very long rubber band (played back in slow motion). At first release starting from a velocity of 0 and then accelerating. but after expending it's energy, slowing? heck, then even retracting?

in other words, what evidence supports that this thing is going to expand at an accelerating rate forever? seems like gravity is going to get a little upset about that eventually.

The amount of gravitational energy between two objects is a static amount that can be determined. The amount of energy in kenetic motion can also be determined. As two objects move apart the gravetational potential grows while the speed they are traveling away from eachother decreases. If the kenetic energy is greater than the gravatational energy, then the two objects will continue to move apart. If the gravetational energy is greater than the kenetic energy causing them to separate, then their realative motion away from eachother will slow, stop and then they will begin to come back together. This is basically an explantion of escape velocity that you always hear about in rocket launches.

A condensed explantion of what would take too long to describe here in full basically says that the average kentic energy that the obejcts in the universe has been determined as well as the average gravatational energy. If the gravetatinal energy was greater then everything would eventually come back together in what is known as a "big crunch", but the kenetic energy is greater and thus the universe will continue to expand.

Now part of this probelms comes not from actual kenetic energy, but due to that space itself is increasing. So the distance between two objects is increasing proportional to the distance between them. the rate at which this is occuring also seems to be growing. This is the acceleration of the universe you are reading about here. Reasons for this are nt well known, but one theory is that there are something like 11 total dimentions and the other 7 after three spacial and time are shrinking, causing the others to expand. These other seven are already so small that they haven't been detected yet (we're way off in string theory territory here).

If this acceleration continues to increase, then eventually the rate at which space is expanding might grow so large that it will overcome not only gravity but even the other forces that hold atoms or particles together. This senario where everything is torn apart into component parts is called the "big rip".

in other words, what evidence supports that this thing is going to expand at an accelerating rate forever? seems like gravity is going to get a little upset about that eventually.

Which is exacly why scientists have postulated the existance of dark energy. You see, you're correct, the effect of gravity does suggest that the universe's expansion should be decelerating. But it's not. All of our observations say that it's accelerating. Most cosmologists would say that's it's pretty much a confirmed fact at this point. The cause of this acceleration is unknown. They're postulaing the existance of this 'dark enery' which exerts some sort of repulsive force.

Enistien actually came up with the idea first, but for a completely different reason. He didn't call it dark enegry though (I don't think), it was just a variable that he added to his equations to force the overall 'shape' of the universe's space-time to be flat. He later took it out because he thought it was stupid; it was much more logical to assume the universe wasn't flat, in which case it wasn't needed. However modern day measurements of the Cosmic Background Radiation [uchicago.edu] have given very strong evidence that the universe is actually flat. So now they've put the variable back into his equations, and they're working on trying to prove it's existence.

clusters are filled with hot gas that emit X-rays with a spectrum indicative of their temperature (typically a million Kelvins or so)

the X-ray luminosity depends on the temperature and the gas mass

the temperature depends on the total (gas + dark) mass

Chandra measures the spectrum which gives you the temperature) and the flux (luminosity / distance^2)

therefore you can find the distance given the gas/dark mass ratio

because clusters are really big and sample a big fraction of the universe, the gas/dark mass ratio is typical of the universe as a whole... and more importantly, that means that all big clusters have the same gas/dark mass ratio

setting the gas/dark mass ratio of all 26 clusters equal gives you the ratio of all of their distances

measuring the redshift of the galaxies in the clusters gives you a relationship between the rate of expansion and distance (relative to the nearest cluster, say)

when you look at this diagram, you see that as things get farther away, the expansion rate increases... and then if you get really far away, it decreases again. this is exactly consistent with what you expect from the cosmological constant (or any form of dark energy with a similar equation of state)

W/ regards to dark mass isn't a more plausible explanation one that just hypothesizes that the x-ray luminosity curves are wrong? The response to this "well there are other things that tell us there is dark mass i.e. i've heard is the "nuclear cycle" tells us how much "regular" mass given the age of the universe is way off. Is not not plausible that something is off in that calculation?

is 1689. You can see an awesome picture of it HERE. [nasa.gov] It's about two billion light years away and one of the most massive objects in the Universe. It's so massive that those blue arcs in the picture are actually galaxies that are being visually warped by the gravity lensing. The amount of matter required to warp space that much is about 99 percent more than we can see in that image.

>everything it states about ratios and measurements involves assumptions. This isn't science.

Yes, it is science. There are observations made that are attempting to confirm or disprove predicitons made consistent with their hypotesis. As for your distaste for the choice of language, particularly the weasle words; that's the way scientists write.

"Recent observations of a massive shockwave, intense gamma, beta, and alpha radiation, together with so far unrepeated visual observations of what is thou

...God's chosen people. Palestinians are just reaping the results of working against God's will and worshipping pagan gods.

Heh, and you still wonder why people hate fundamentalist nutters like you. It's weird that fundamentalist muslims and orthodox jews are at each others' throats... You have more in common then you are different